In a nutshell I think it's a matter of "have a taste of your own medicine" at Ukraine. In 2008 a Georgian SA-11 battery, an earlier version of the same missile platform, was responsible for the loss of a Russian Tu-22 and three Su-25s.
Georgia had bought that SA-11 battery from Ukraine. The Russians were rather displeased and raised a huge stink about the sale of the missile platform to Georgia being potentially illegal.
Suddenly it's much less surprising that there might be SA-17 or SA-11 batteries either in the hands of Ukrainian rebels (some few of whom are likely ex-military themselves and could well include folks capable of operating the battery) or being driven around the Ukrainian countryside by Russians. Personally, I would lean more towards the former scenario because I would have thought that Russian regulars would be less likely to be stupid enough to fire upon an airliner.
Rebel irregulars on the other hand.. well they might just be ill-trained enough not to recognize that their target was acting like an airliner or crazy enough not to care. Either way this is going to get real ugly for Russia and there's going to be a lot of spin and damage-control efforts over the next few weeks.